The present invention relates to an image output device, such as a laser printer and to a test chart for calibration of tone reproduction characteristics in the image output device, and also concerns a method and a device of calibration using the test chart.
The electrophotographic type printer forms an electrostatic latent image by radiating a laser beam to a photosensitive body, and neutralizes the latent image by a toner oppositely charged with respect to the polarity of the latent image.
As disclosed in JP-A-07-128974, because the quantity of electric charge on the toner particles varies with environmental conditions, such as temperature and humidity, the density of a reproduced image changes with environmental conditions. For example, when temperature and humidity are high, electric charge per unit toner particle decreases, and on the other hand, when temperature and humidity are low, electric charge per unit toner increases.
It is a general tendency of electrophotographic printers that under high temperature and humidity, compared with low temperature and humidity, a problem rises that a larger amount of toner is required for the above-mentioned neutralization and therefore reproduced images have high density.
Further, with electrophotographic printers, it is widely known that there are instrument errors, in other words, variation occurs in the development characteristics among different machines due to assembly tolerances related to laser spot diameter and laser intensity-sensitive material characteristics, and that the development characteristics change with time.
The above-mentioned variation and changes with time are corrected by high-level feedback and also by calibration by service persons using a calorimeter or the like, but those methods contribute to increases in production and running costs.
As countermeasures against changes mentioned above, there have been proposed a method disclosed in JP-A-07-128974 mentioned above, in which the developing machine is controlled by a toner densitometer and temperature and humidity sensors, and another method disclosed in JP-A-10-6562, in which a color printer is calibrated by comparison with a test chart, with which a halftone patch previously printed to a reference density.
There is growing demand for color printers from general users with widespread use of personal computers. To meet this demand, it is required that the color printers be made available at lowest possible prices.
If color laser printers are equipped with a toner densitometer or temperature and humidity sensors as suggested in JP-A-07-128974, this will lead to higher prices. In the method of calibrating a color printer by comparison with a test chart using a halftone patch printed in advance to a reference density as disclosed in JP-A-10-6562, know-how or experience is required to achieve desired precision and this method is difficult for general users to use.
When the prior art in JP-A-07-128974 is used, in which the relation between specified tone values and photographic printing density (or lightness or saturation) is not necessarily linear, in methods of controlling toner density or maximum laser intensity, even though being effective in stabilizing the density (or lightness or saturation) of the solid (shadow) areas of reproduced images, these methods have a problem that the density (or lightness or saturation) of halftones cannot be made stable.
In the prior art in JP-A-10-6562, because discoloring occurs in the colors in reference charts, calibration references change with time, it has been difficult to maintain desired precision in calibration. The user must carefully manage those charts, a fact which, as well as the discoloring problem, has been a burden on the user while he or she is using related equipment.